Effects of confinement on relaxation in glass-formers: Magnitude, trends and dependence on the Kohlrausch exponent

We show that the Adam-Gibbs theory cannot by itself explain the reduction o f tau(alpha) and T-g when a supercooled liquid is confined in nano-meter po res. However, when combined with the coupling model, the modified theory ca n explain the finite-size effects of small molecule glass-formers in pores and ultra thin polymer films. The modified theory also predicts two additio nal behaviors. First, magnitude of the finite-size effect is larger in liqu ids that have smaller exponent, beta, in the Kohlrausch expression for the structural relaxation of the unconfined liquid at T-g. Second, the dependen ce of the structural relaxation time of the liquid on T-g/T becomes increas ingly weaker with decreasing pore size. These predicted behaviors are found in the experimental data. A previously proposed mechanism for the glass tr ansition reduction in thin freely-standing polymer films [K.L. Ngai et al., J. NonCryst. Solids 235-237, 435 (1998)] is revisited. Recent Monte Carlo simulation results support the mechanism.